Today, the two leading types of printers on the market are laser printers and inkjet printers. In general, laser printers utilize toner to produce images, while inkjet printers produce images by depositing ink.
Laser printers and inkjet printers have major differences in performance and price. In general, laser printers are significantly faster and have a greater printing duty cycle than inkjet printers. In addition, laser printers typically have significantly lower operating costs than inkjet printers. However, inkjet printers are capable of producing photo-quality images at higher resolutions than laser printers. And inkjet printers are typically significantly less expensive than color laser printers.
As a result, most of the printers on the market are monochrome laser printers and color inkjet printers. Office computer networks typically include a monochrome laser printer for large-volume and rapid printing, and typically use a color inkjet printer for the occasional color print job. However, this approach not only involves the expense of purchasing at least two separate printers, but also the expense of supporting and maintaining the at least two separate printers. There are few, if any, printers available that have the speed and duty cycle of a laser printer as well as the color and resolution of an inkjet printer at a reasonable price.
FIG. 1 is a diagram of a typical laser printer 10 including a movable photoreceptor 12, typically a revolving drum or cylinder. This drum 12 is made out of a highly photoconductive material that is discharged by light photons. Initially, the photoreceptor drum 12 is given a total positive charge by a charging electrode 14, typically a wire or roller having a current running through it. As the drum 12 revolves, the printer 10 uses a laser unit 16 (such as a laser diode) to shine a laser beam 18 across the surface of the drum 12 to discharge certain points. In this manner, the laser beam 18 “draws” the text and images to be printed as a pattern of electrical discharges (an electrostatic image) on the drum 12. If the on-time or intensity of the laser beam 18 is modulated, resulting variations in charge on the drum 12 will ultimately be translated to proportionate amounts of toner deposited on a print medium 21 such as paper.
In scanning the laser beam 18 across the drum 12, the laser unit 16 does not actually move the laser beam 18 itself but instead bounces the laser beam 18 off of a movable mirror 26, such as a rotating mirror or an oscillating mirror. As the mirror 26 moves, it reflects the laser beam 18 through a series of lenses (not shown) and onto the drum 12. These lenses change characteristics of the light beam 18 to compensate for image distortion that would otherwise be caused by the varying distance between the mirror 26 and points along the drum 12.
After the laser beam 18 begins scanning the desired electrostatic pattern on the drum 12, the printer 10 uses a toner roller 20 to coat the drum 12 with positively charged toner powder. Since the toner has a positive charge, it clings to the negative discharged areas of the drum 12 that have been scanned by the laser beam, but the toner does not cling to the positively charged “background” of the drum. With the toner pattern affixed to the drum 12, the drum rolls over the sheet of paper 21 traveling below it. Before the paper 21 travels under the drum 12, the paper is given a negative charge that is stronger than the negative charge of the electrostatic image on the drum 12 so that the paper pulls the toner powder away from the drum 12. Finally, the printer 10 passes the paper 21 through a fuser 24, which is typically a pair of heated rollers. As the paper 21 passes through the fuser 24, the loose toner powder on the paper melts, fusing with the fibers in the paper and forming a permanent image on the paper.
After the toner on the drum 12 is transferred to the paper 21, the drum surface rotates past a discharge lamp 22, which generates a bright light that exposes the photoreceptor surface of the drum 12, erasing the electrostatic image. The drum surface then passes the charging electrode 14, which reapplies a positive charge to the surface of the drum 12 in anticipation of the laser beam 18 scanning the next portion of the image to be printed onto the drum. In this way, the scanning of the laser, the transfer of the toner, and the erasing of the photoreceptor surface is a continuous process that may be repeated before the printing of an entire image is complete.
FIG. 2 is a perspective view of portions of the laser printer 10 in FIG. 1 better showing the scanning of the laser beam 18 in a horizontal direction across the drum 12 as indicated by an arrow 28. Image processing circuitry (not shown) controls the laser unit 16 to modulate the laser beam 18 as the beam scans across the drum 12 in the horizontal direction 28 one line at a time. The image processing circuitry controls the laser unit 16 to turn ON and emit a pulse of light for every dot to be printed in a given horizontal line and to turn OFF where no dots are to be printed in the line.
Laser printers typically have several advantages when compared to other types of printers. For example, laser printers often produce documents with the highest text and line quality. This is because laser printers use toner, which sits crisply on top of the paper and does not spread like ink does as it is absorbed by the paper. Also, the cost of operating a laser printer is typically low; certain toner cartridges have average yields of over 40,000 pages. This allows laser printers to have a cost per page as low as one cent. In addition, laser printers are often designed to print pages at a high rate; current laser printers can print black-and-white documents at speeds of up to 85 pages per minute (ppm).
Laser printers do, however, have certain limitations. To print in color, laser printers typically have a separate toner cartridge for each of the primary colors. Consequently, instead of a single black toner cartridge, a color laser printer often has four toner cartridges (e.g., black, red, yellow, blue). In addition, the architecture of a color laser printer is often significantly more complex than the architecture of a black-and-white laser printer. For example, the four toner cartridges either share the same drum so that four passes of the paper are made over the same drum, or have their own respective drums so that the paper passes over four separate drums in series. As a result, the cost of a color laser printer is typically higher than the costs of other types of color printers. Also, because overlapping toner colors may reduce image resolution and quality, color laser printers often produce color images that are lower in resolution and quality than comparable color images produced by other types of color printers.
FIG. 3 is a perspective view of a typical inkjet printer 30 that emits droplets of ink (not shown) onto print media 32, such as paper, to create images and text. The inkjet printer 30 includes a printhead (not shown) mounted within a carriage 34 that travels back and forth across the paper 32. The printhead includes an array of tiny nozzles (not shown) that emit the droplets of ink. As the printhead moves across the paper 32, a controller activates the printhead to emit droplets of ink at precise locations corresponding to a pattern of pixels of the image being printed.
Ink is typically provided to the printhead by an ink cartridge 36 that is attached to the carriage 34. Depending on the design of the printer 30, the ink cartridge 36 can come in various combinations ranging from a single cartridge for black ink to multiple cartridges each for an ink of a desired color. The ink cartridge 36 may even include the printhead itself. Alternatively, the ink cartridge 36 may be a stationary ink reservoir that is separate from the carriage 34 and connected to the printhead by a hose.
Different types of inkjet printers emit the droplets of ink in different ways. The two main inkjet technologies currently used by printer manufacturers are thermal bubble (also known as bubble jet) and piezoelectric.
FIG. 4A is a cross-sectional view of a typical bubble jet printhead 40 used in a thermal inkjet printer. In the printhead 40, a heating element 41 such as a resistor creates heat that vaporizes ink in a reservoir 42 to create a bubble. As the bubble expands, a tiny amount of the ink is emitted from a nozzle 43 onto the paper. When the bubble collapses, a vacuum is created that pulls more ink into the reservoir 42 from the ink cartridge 36 (FIG. 3).
FIG. 4B is a cross-sectional view of a typical piezoelectric printhead 45 used in a piezoelectric inkjet printer. In the printhead 45, a transducer 46 such as a piezo crystal is located at the back of an ink reservoir 47. The transducer 46 receives an electric signal that causes it to vibrate. When the transducer 46 vibrates inward toward the reservoir 47, it forces a tiny amount of ink out of a nozzle 48 onto the paper. When the transducer 46 vibrates outward away from the reservoir 47, it pulls more ink into the reservoir 47 from the ink cartridge 36.
Color inkjet printers typically have several advantages when compared to other types of printers. For example, color inkjet printers are often capable of high resolutions. Current inkjet printheads can emit droplets of ink as small as 2 picoliters. This allows such printheads to produce images with resolutions exceeding 4800 dots per inch (dpi). In addition, color inkjet printers can have up to eight color ink cartridges, each having a different color of ink. This significantly broadens the range of colors produced by the printhead, and allows the printhead to produce photo-quality images that change tone gradually without discernable patterns or jumps in color. Also, the cost of color inkjet printers is significantly lower than the cost of other types of color printers.